The mixed-valence heteropolyanion PMo12O40n- (where n = 4, 5, 6, 7) in aprotic media was investigated by cyclic voltammetry and in-situ FTIR and UV-visible-near-IR spectroelectrochemical methods. A new optically transparent thin layer cell with adjustable optical path length was designed and characterised by thin layer cyclic voltammetry and double step chronoamperometry using ferrocene as a test redox system. The results indicate that this cell has small ohmic drop and good spectral response. The cell was used for both in-situ FTIR and UV-visible-near-IR spectroelectrochemical measurements. The cyclic voltammetric results indicate that the heteropolyanion PMo12O403- undergoes two reversible one-electron transfer reductions (first and second redox waves) and two quasi-reversible one-electron transfer reductions (third and fourth redox waves). The mixed-valence heteropolyanion PMo12O40n-(where n = 4, 5, 6, 7) was formed after electroreduction. In-situ FTIR and UV-visible-near-IR spectroelectrochemical preliminary results indicate that the electronic structure of electrogenerated mixed-valence species PMo12O404- corresponds to the class II system in Robin and Day＇s classification of mixed-valence compounds. In-situ FTIR spectroelectrochemical studies also suggest that the bond energy of the Mo=O-d double bond and Mo-O-b-Mo and Mo-O-c-Mo bridge bonds was reduced after reduction of the original compound.